A man dressed as a Roman soldier in a file photo. (AP Photo/Hassan Ammar)

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Reading ancient battlefields is no easy task given they so often leave no trace behind—wood disintegrates, iron rusts, and stones aren't detectable. But lead is different, and a cache of hundreds of ancient sling bullets was readily detected thanks to specialized metal detectors in Burnswark Hill in southern Scotland. Their discovery tells the story of what may be among the first battles of the Roman invasion of Scotland (then called Caledonia) around AD140, reports the BBC. It's a big break for battlefield archaeologists who have been trying for centuries to sort out the precise role Burnswark Hill played in Rome's attempted occupation of the area 1,800 years ago. And it seems the Romans staged a huge show of force to overtake an outnumbered enemy guarding a fort.

The excavations turned up the largest cache of Roman sling bullets ever found, from small ones drilled with holes that would have made terrifying whistling sounds to much larger lemon-shaped ones that were "very lethal," one archaeologist tells Live Science. Researchers estimate that there were 5,000 Roman soldiers spread across two army camps and as many as 2,000 armed defenders. The ensuing violence was likely "exemplary," an "opening event" meant to terrorize Caledonian tribes as Romans tried to push north of Hadrian's Wall, which was the upper border of the Roman Empire. (Here's an even older battlefield find.)